1. Field of the Invention
This invention relates generally to nucleic acid sequences encoding polypeptides that are associated with abiotic stress responses and abiotic stress tolerance in plants. In particular, this invention relates to nucleic acid sequences encoding polypeptides that confer upon the plant increased growth and/or that confer increased drought, cold, and/or salt tolerance to plants.
2. Background Art
Abiotic environmental stresses, such as drought stress, salinity stress, heat stress, and cold stress, are major limiting factors of plant growth and productivity. Crop losses and crop yield losses of major crops such as soybean, rice, maize (corn), cotton, and wheat caused by these stresses represent a significant economic and political factor and contribute to food shortages in many underdeveloped countries. Plants are typically exposed during their life cycle to conditions of reduced environmental water content. Most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on development, growth, and yield of most crop plants are profound. Continuous exposure to drought conditions causes major alterations in the plant metabolism, which ultimately lead to cell death and consequently yield losses.
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies to develop new lines of plants that exhibit resistance (tolerance) to these types of stresses are relatively slow and require specific resistant lines for crossing with the desired line. Limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to drought, cold, and salt tolerance in model drought-, cold-, and/or salt-tolerant plants are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways. This multi-component nature of stress tolerance has not only made breeding for tolerance largely unsuccessful, but has also limited the ability to genetically engineer stress tolerant plants using biotechnological methods.
Drought stresses, heat stresses, cold stresses, and salt stresses have a common theme important for plant growth and that is water availability. As discussed above, most plants have evolved strategies to protect themselves against conditions of desiccation; however, if the severity and duration of the drought conditions are too great, the effects on plant development, growth and yield of most crop plants are profound. Furthermore, most of the crop plants are very susceptible to higher salt concentrations in the soil. Because high salt content in some soils results in less water being available for cell intake, high salt concentration has an effect on plants similar to the effect of drought on plants. Additionally, under freezing temperatures, plant cells lose water as a result of ice formation that starts in the apoplast and withdraws water from the symplast. A plant's molecular response mechanisms to each of these stress conditions are common, and protein kinases, such as lectin-like protein kinases, play an essential role in these molecular mechanisms.
Protein kinases represent a superfamily, and the members of this superfamily catalyze the reversible transfer of a phosphate group of ATP to serine, threonine, and tyrosine amino acid side chains on target polypeptides. Protein kinases are primary elements in signaling processes in plants and have been reported to play crucial roles in perception and transduction of signals that allow a cell (and the plant) to respond to environmental stimuli.
One type of protein kinase is the lectin-like protein kinase (LLPK) or lectin receptor kinase. Structural features of this type of protein kinase include an amino-terminal membrane-targeting signal sequence, a legume lectin-like extracellular domain, a single membrane-spanning domain, and a characteristic serine/threonine protein kinase catalytic domain. Members of this family have been reported to be involved in cell-cell communication, defense against predators and pathogens, and plant development and reproduction (Barre et al., 2002, Crit. Rev. Plant Sci. 21:379-399). Forty-two putative lectin receptor kinases and nine soluble legume lectin sequences have been identified in Arabidopsis. 
Although some genes that are involved in stress responses and water use efficiency in plants have been characterized, the characterization and cloning of plant genes that confer stress tolerance and water use efficiency remains largely incomplete and fragmented. For example, certain studies have indicated that drought and salt stress in some plants may be due to additive gene effects, in contrast to other research that indicates specific genes are transcriptionally activated in vegetative tissue of plants under osmotic stress conditions. Although it is generally assumed that stress-induced proteins have a role in tolerance, direct evidence is still lacking, and the functions of many stress-responsive genes are unknown.
There is a fundamental physiochemically-constrained trade-off, in all terrestrial photosynthetic organisms, between CO2 absorption and water loss (Taiz and Zeiger 1991 Plant Physiology, Benjamin/Cummings Publishing Co, p 94). CO2 needs to be in aqueous solution for the action of CO2 fixation enzymes such as Rubisco (Ribulose 1,5-bisphosphate Carboxylase/Oxygenase) and PEPC (Phosphoenolpyruvate carboxylase). As a wet cell surface is required for CO2 diffusion, evaporation will inevitably occur when the humidity is below 100% (Taiz and Zeiger 1991 Plant Physiology, Benjamin/Cummings Publishing Co p 257). Plants have numerous physiological mechanisms to reduce water loss (e.g. waxy cuticles, stomatal closure, leaf hairs, sunken stomatal pits). As these barriers do not discriminate between water and CO2 flux, these water conservation measures will also act to increase resistance to CO2 uptake (Kramer 1983 Water Relations of Plants, Academic Press p 305). Photosynthetic CO2 uptake is absolutely required for plant growth and biomass accumulation in photoautotrophic plants. Water Use Efficiency (WUE) is a parameter frequently used to estimate the trade off between water consumption and CO2 uptake/growth (Kramer 1983 Water Relations of Plants, Academic Press p 405). WUE has been defined and measured in multiple ways. One approach is to calculate the ratio of whole plant dry weight, to the weight of water consumed by the plant throughout it's life (Chu et al 1992 Oecologia 89:580). Another variation is to use a shorter time interval when biomass accumulation and water use are measured (Mian et al 1998 Crop Sci. 38:390). Often measurements from restricted parts of the plant are used, for example, measuring only aerial growth and water use (Nienhuis et al 1994 Amer J Bot 81:943). WUE has also been defined as the ratio of CO2 uptake to water vapor loss from a leaf or portion of a leaf, often measured over a very short time period (seconds/minutes) (Kramer 1983 Water Relations of Plants, Academic Press p 406). The ratio of 13C/12C fixed in plant tissue, and measured with an isotope ratio mass-spectrometer, has also been used to estimate WUE in plants using C3 photosynthesis (Martin et al 1999 Crop Sci. 1775).
An increase in WUE is informative about the relatively improved efficiency of growth and water consumption, but on it's own it doesn't describe which of these two processes (or both) have changed. In selecting traits for improving crops, an increase in WUE due to a decrease in water use, without a change in growth would have particular merit in an irrigated agricultural system where the water input costs were high. An increase in WUE driven mainly by an increase in growth without a corresponding jump in water use would have applicability to all agricultural systems. In many agricultural systems where water supply is not limiting, an increase in growth, even if it came at the expense of an increased water use (i.e. no change in WUE), could also increase yield. Therefore new methods to increase both WUE and biomass accumulation are required to improve agricultural productivity. As WUE integrates many physiological processes relating to primary metabolism and water use, it is typically a highly polygenic trait with a large genotype by environment interaction (Richards et al 2002 Crop Sci 42:111). For these and other reasons few attempts to select for WUE changes in traditional breeding programs have been successful.
There is a need, therefore, to identify genes expressed in stress tolerant plants and plants that are efficient in water use that have the capacity to confer stress tolerance and water use efficiency to its host plant and to other plant species. Newly generated stress tolerant plants will have many advantages, such as an increased range in which the crop plants can be cultivated, by for example, decreasing the water requirements of a plant species. Other desirable advantages include increased resistance to lodging, the bending of shoots or stems in response to wind, rain, pests, or disease.